When there is an issue with booting up RHEL / CentOS 7 system you can boot it into rescue mode using installation DVD or ISO to troubleshoot the issue. You can have the access to the disk where the OS is installed, to have a look at the problem or do a recovery from a corrupt kernel etc.
Booting into rescue mode using DVD/ISO
Follow the steps below to boot the system into rescue mode using installation media such as DVD or ISO.
1. Attach the ISO image
You can use an actual installation DVD instead of ISO image, but I find using ISO image easy and there is no need to go to the data center to physically insert the DVD into the system. Different virtualization platforms have similar features to attach/mount the ISO image to a VM guest. Make sure you change the boot order to boot from the ISO image.
2. Boot up the system
Boot up the CentOS 7 system from ISO image. At the boot screen, Select the Troubleshooting option at the end of the screen.
3. At the next screen, select the option Rescue a CentOS Linux system.
4. On the next screen, press enter to continue. When asked if you would like Rescue to find your installation, choose Continue.
If you run into trouble detecting your install, retry using the Skip option and manually detect and mount your storage. You would get a message shown in the picture below if the rescue mode has detected the correct installation.
5. Next step is to change your root directory to /mnt/sysimage using the chroot command. This makes your system the root environment.
# chroot /mnt/sysimage